It is known that the wear parts of sliding gate valves, i.e. closure units, are subjected to extraordinarily high erosive and/or corrosive attacks. Such wear parts presently are formed of very high value ceramic refractory materials which, however, are subject to unsatisfactorily high wear and tend to the formation of cracks at extreme thermal stresses. There is further the danger with such sliding gate valves of conventional type of the freezing of the metal in the through passages when the sliding gate valve is closed. These disadvantageous phenomena depend in part on the thermal conductivity and insulating properties of the refractory material of the wear part.